The present invention relates to a small shape recognizing capacitive sensor device and, more particularly, to a small shape recognizing capacitive sensor device for sensing a small three-dimensional pattern such as a human fingerprint or animal noseprint.
As application examples of a small shape recognizing capacitive sensor device, a number of fingerprint sensors for detecting a fingerprint pattern have been proposed (e.g., xe2x80x9cISSCC DIGEST OF TECHNICAL PAPERSxe2x80x9d February 1998, pp. 284-285). With this technique, an electrostatic capacitance formed between sensor electrodes in cells (to be referred to as sensor cells hereinafter) which are two-dimensionally arrayed on an LSI chip and the skin of a finger which comes into contact with the sensor electrodes via a passivation film is detected, thereby sensing the three-dimensional pattern on the finger skin surface. Since the value of the formed capacitance changes depending on the three-dimensional pattern on the finger skin surface, the three-dimensional pattern on the finger skin surface can be sensed by detecting the capacitance difference.
FIG. 36 shows sensor cells of such a conventional small shape recognizing capacitive sensor device. Each sensor cell 11 is formed from a detection element 1 and sensor circuit 2. The detection element 1 is an element for converting the surface shape into an electrical signal. The sensor circuit 2 is a circuit for measuring the electric change amount from the detection element, which changes depending on the surface shape. An output signal 2A output from the sensor cell 11 is input to an A/D conversion circuit 4 through a data line LD and output as a digital output signal 4A. The data line LD is shared by the plurality of sensor cells 11. The sensor cells 11 are sequentially selected, and the output signals 2A from the sensor cells 11 are sequentially input to the A/D conversion circuit 4.
FIG. 37 shows the detailed structure of the conventional sensor cell. The detection element 1 is an element for converting the surface shape into an electrical signal 1A. The sensor circuit 2 is a circuit for measuring the electric change amount from the detection element 1, which changes depending on the surface shape. The detection element 1 is implemented by a sensor electrode 1B formed on an insulating layer 16 and covered with a passivation film 15 and uses, as the electrical signal, an electrostatic capacitance CF formed between a finger skin 14 and the sensor electrode 1B. The sensor circuit 2 is formed from a Pch MOSFET Q1, Nch MOSFETs Q2 and Q3, constant current source I, and resistor R. CP0 a is a parasitic capacitance.
FIG. 38 shows the operation timing. Before time T1, a sensor circuit control signal PRE0 is controlled to a power supply voltage VDD to keep the MOSFET Q1 off, and a sensor circuit control signal RE is controlled to a voltage of 0 V to keep the MOSFET Q2 off. A node N1 is at 0 V. At time T1, the signal PRE0 is controlled to 0 V to turn on the MOSFET Q1, and the node N1 rises to VDD. At time T2, the signals PRE0 and RE are controlled to VDD to turn off the MOSFET Q1 and turn on the MOSFET Q2. Charges accumulated in the electrostatic capacitance CF are removed, and the potential of the node N1 gradually drops. At time T3 later than time T2 by xcex94t, when the signal RE is controlled to 0 V to turn off the MOSFET Q2, the potential VDDxe2x88x92xcex94V of the node N1 at that time is kept and output from the MOSFET Q3. With this operation, the output signal 2A having a voltage corresponding to the value of the electrostatic capacitance CF is generated. The three-dimensional pattern on the skin surface can be detected by measuring the magnitude of the voltage signal.
In such a conventional small shape recognizing capacitive sensor device, however, although the sensor cells are manufactured according to the same layout, the actual sensor circuits 2 do not have completely same detection sensitivity due to a variation in process. As a result, the detected image has noise due to a variation in sensitivity between the sensor circuits, and the image quality degrades. In addition, sensors having poor detection performance are formed by a variation between chips or a variation between wafers. This decreases the yield of sensor chips, resulting in an increase in manufacturing cost. This poses a serious problem especially for supply of inexpensive sensor chips.
Furthermore, even a sensor having satisfactory detection performance degrades its detection performance when the sensor surface changes depending on the use state. This shortens the performance guarantee period, and the module incorporating the sensor becomes unusable. In a system using sensors, the sensor components must be often exchanged. For this reason, the cost for coping with returned products or system maintenance increases, resulting in a serious problem. Hence, in the conventional small shape recognizing capacitive sensor device, the manufacturing cost or maintenance cost increases because the device has no means for individually adjusting the detection sensitivities of the plurality of sensor circuits.
It is the principal object of the present invention to provide a small shape recognizing capacitive sensor device having higher yield than the prior art.
In order to achieve the above object, according to the present invention, there is provided a small shape recognizing capacitive sensor device comprising a number of detection elements arranged adjacent to each other, a number of sensor circuits connected to the detection elements, respectively, and a correction circuit for correcting an output signal level of the sensor circuit, the output signal level correction circuit comprising a calibration circuit connected to an output side of the sensor circuit, a calibration reference signal generation circuit for generating a calibration reference signal, and a comparison circuit for comparing an output from the sensor circuit with the calibration reference signal and supplying a difference output to the calibration circuit as a control signal, wherein the calibration circuit corrects a level of a sensor circuit output on the basis of the control signal such that the difference between the output from the sensor circuit and the calibration reference signal becomes zero.